Engine and Drivetrain Systems
Engine and Drivetrain Systems
This domain tests how a light vehicle converts fuel into torque and delivers it to the road: four-stroke operation, air and fuel metering, ignition or compression ignition, timing, lubrication and cooling, and the clutch, gearbox, final drive and differential.
Core rules
- Four-stroke cycle: intake, compression, power, exhaust. Valve timing must match piston position; a slipped timing belt or chain can cause low compression, poor running or valve damage.
- Spark-ignition engines meter air and fuel and ignite with a spark; compression-ignition engines inject fuel near top dead centre and rely on high compression for auto-ignition.
- Drivetrain ratios multiply torque but reduce speed. Overall ratio = gearbox ratio × final drive ratio.
- A differential allows wheels to turn at different speeds in a turn while transmitting torque.
- Low compression, incorrect valve clearance and restricted intake or exhaust can all mimic an ignition fault.
Worked example
A four-cylinder engine has a bore of 86 mm and a stroke of 86 mm. Displacement per cylinder = π/4 × bore² × stroke. Convert to centimetres: 8.6 cm. π/4 × 8.6² × 8.6 = 0.7854 × 73.96 × 8.6 ≈ 499.6 cm³. Four cylinders give about 1,998 cm³, or 2.0 L. Use the π/4 form consistently, keep units in centimetres, and multiply by cylinder count only at the end.
Traps
- Confusing bore with stroke when calculating displacement.
- Assuming a timing mark is correct without checking the manufacturer's alignment procedure.
- Diagnosing a misfire as ignition when a vacuum leak, low compression or a faulty injector is the cause.
- Forgetting that tyre diameter changes the effective final drive and speedometer reading.